Pressure sensors have been the interest from the earliest development of microelectromechanical systems (MEMS). However, at that point, the majority of MEMS sensors were assembled by combining separately manufactured mechanical, e.g. silicon (Si) MEMS, and electrical components, e.g. complementary oxide semiconductor (CMOS) integrated circuits (IC) for low power operation, formed upon two different substrates. These discrete MEMS and CMOS components were then used to realize the final pressure sensor by mounting them to a common carrier, electrically connecting the MEMS and CMOS parts together through wire-bonding or other assembling technologies, and then packaging them. This approach to integration imposes limitations on performance, e.g. due to the higher parasitics involved, on size, cost, etc.
Accordingly, there has been interest within the industry for tighter integration of the MEMS and IC parts, as this should mitigate many of the limitations identified above and is a logical solution to enhance the functionality of systems. This has led to a variety of techniques being developed and employed within the prior art for CMOS-MEMS integration through both hybrid and monolithic integration, the latter being where both technologies are implemented on a single common substrate during manufacturing. The most common monolithic integration being the formation of the MEMS device(s), after the manufacture of the CMOS circuit, within dedicated areas without electronics. However, such monolithically integrated CMOS-MEMS systems adds complexity to the overall fabrication process sequence due to a variety of factors including, for example, added constraints on the thermal budgets and the materials stresses involved.
As a result, the monolithic integration of MEMS sensors with CMOS overall has proven to be a challenging task, and with respect to MEMS pressure sensors with CMOS IC's then these have proven to be a very challenging task. This is primarily due to the need for such devices to include a very high quality thin diaphragm to provide the pressure dependent MEMS deformation that can be sensed and, when seeking absolute rather than relative pressure sensors, a sealed reference cavity. Typically, this is achieved within the prior art through a bonding process that employs multiple wafers, or by exploiting high temperature deposition techniques capable of creating the required movable/deformable diaphragm. However, such high temperature deposition techniques are not compatible with standard CMOS processes, i.e. the temperatures involved are typically higher than 400° C., thereby requiring that the CMOS be fabricated after the formation of the MEMS, which is extremely challenging or that the CMOS electronics are degraded.
Accordingly, the inventors have established a new manufacturing process based upon back-etching and bonding of a monolithic absolute silicon carbide (SiC) capacitive pressure sensor. Beneficially, the process achieves integration through the embedding of the critical features of the MEMS within a shallow trench formed within the silicon substrate and then processing the CMOS circuit. Accordingly, the process beneficially maintains that the components of the MEMS element fabrication process that are CMOS compatible are implemented concurrently with those CMOS steps as well as the metallization steps. However, the CMOS incompatible processing is partitioned discretely.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.